Early detection of pregnancy-related conditions, including potential complications during pregnancy or delivery and genetic defects of the fetus is of crucial importance, as it allows early medical intervention necessary for the safety of both the mother and the fetus. Prenatal diagnosis has been routinely conducted using cells isolated from the fetus through procedures such as chorionic villus sampling (CVS) or amniocentesis. These conventional methods are, however, invasive and present an appreciable risk to both the mother and the fetus despite most careful handling (Tabor et al., Lancet 1:1287-1293, 1986).
Alternatives to these invasive approaches have been developed for prenatal screening, e.g., to detecting fetal abnormalities, following the discoveries that several types of fetal cells can be found in maternal circulation (Johansen et al., Prenat. Diagn. 15:921-931, 1995) and more importantly, circulating cell-free fetal DNA can be detected in maternal plasma and serum (Lo et al., Lancet 350:485-487, 1997). The amount of fetal DNA in maternal blood has been shown to be sufficient for genetic analysis without complex treatment of the plasma or serum, in contrast to alternative methods requiring steps for isolating and enriching fetal cells. Fetal rhesus D (RhD) genotyping (Lo et al., N. Engl. J. Med. 339:1734-1738, 1998), fetal sex determination (Costa et al., N. Engl. J. Med. 346:1502, 2002), and diagnosis of several fetal disorders (Amicucci et al., Clin. Chem. 46:301-302, 2000; Saito et al., Lancet 356:1170, 2000; and Chiu et al., Lancet 360:998-1000, 2002) have since been achieved by detecting fetal DNA in maternal plasma or serum using a polymerase chain reaction (PCR)-based technique.
In addition, quantitative abnormalities of fetal DNA in maternal plasma/serum have been reported in preeclampsia (Lo et al., Clin. Chem. 45:184-188, 1999 and Zhong et al., Am. J. Obstet. Gynecol. 184:414-419, 2001), fetal trisomy 21 (Lo et al., Clin. Chem. 45:1747-1751, 1999 and Zhong et al., Prenat. Diagn. 20:795-798, 2000) and hyperemesis gravidarum (Sekizawa et al., Clin. Chem. 47:2164-2165, 2001). Detection of fetal nucleic acid in maternal blood for prenatal genetic analysis is also disclosed in U.S. Pat. No. 6,258,540.
Because fetal DNA co-exists with maternal DNA in the acellular portion of a pregnant woman's blood, e.g., serum or plasma, there is a need to distinguish DNA from fetal origin and maternal origin to ensure accurate results in fetal DNA-based diagnosis. It was first disclosed in U.S. patent application Ser. No. 09/944,951, published as 20030044388, that fetal and maternal DNA may be distinguished by their different methylation profiles. Landes et al. in U.S. Patent Application Publication No. 20030211522 also proposed differential methylation markers may be used for prenatal diagnosis. On the other hand, to ensure the efficacy of fetal DNA-based testing methods and to eliminate erroneous interpretation of test results due to insufficient recovery of fetal DNA obtained from such methods, there also exists a need for determining the presence and quantity of fetal DNA in a sample used for the testing procedure. It is therefore desirable to identify a fetal DNA marker that can effectively serve as a universal indicator of the presence or absence of fetal DNA in general in a test sample. It is important that such a fetal DNA marker is consistently and uniformly distinct from its maternal counterpart, and that the presence or absence of the marker can be readily determined over the background of maternal DNA and directly correlated with the presence or absence of fetal DNA in general. This invention addresses this and other related needs.
In this application, a number of human genes have been identified for the first time as those having highly distinct methylation patterns in fetal tissues (e.g., derived from placenta) and in maternal tissues. Originated from a fetus, these genes are methylated at a high level of uniformity, whereas the genes from a maternal source that releases significant amount of cell-free DNA into the maternal blood are unmethylated at a similarly high level of uniformity. These features allow the genes to effectively serve as internal positive controls of a test sample used in a prenatal diagnostic process, for the purpose of ensuring that a sufficient amount of fetal DNA has been recovered in the sample during the process. Because of the high level of uniformity in these genes' methylation status with regard to their origin, these genes are particularly reliable controls, indicative of both quality and quantity of the fetal DNA. Another advantage of these genes as fetal markers is the relative ease in detecting only the methylated fetal version in contrast to their unmethylated maternal counterparts. Furthermore, the fetal genes described in this application can also be used directly as diagnostic markers for certain conditions or disorders related to pregnancy.